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Atomically dispersed rhodium on a support: the influence of a metal precursor and a support† R. B Duarte,a O. V. Safonova,b F. Krumeicha and J. A. van Bokhoven*ab The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation
Received 12th June 2014, Accepted 15th July 2014
is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over
DOI: 10.1039/c4cp02596b
the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate
supports of the same composition. The stability of rhodium is improved with the addition of promoters; precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive
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treatment at 773 K.
Introduction Numerous catalytic reactions are structure sensitive1 and therefore the precise control of metal dispersion is of ultimate importance. Stable supported atomically dispersed catalysts are highly desirable for heterogeneous catalytic reactions in which s-bond activation is kinetically relevant. A common method for the synthesis of supported metal catalysts is wet impregnation of the support with a metal precursor solution.2 Metal dispersion and properties of catalysts are influenced by the type of support and the nature of the metal precursor: Vasiliadou et al.3 observed higher activity of ruthenium-based catalysts synthesized with RuCl3xH2O in comparison with RuNO(NO3)3 in the hydrogenolysis of biomass due to the retention of Cl ions on the support surface. The dispersion of ruthenium was similar to and independent of the precursor type; only the nature of the oxidic support (alumina, silica and zirconia) was found to influence the dispersion of the metal. In contrast, thermal decomposition of different rhodium precursors (rhodium(II) acetate and rhodium(III) chloride) was reported to yield varied dispersion when supported on alumina.4 Rhodium(II) acetate led to higher dispersion due to the interaction of the metal with oxygen from the support and the use of rhodium(III) chloride favored sintering because of the strength of a
ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. E-mail: [email protected] † Electronic supplementary information (ESI) available: A HAADF-STEM image of a CeO2 crystal supported on the alumina, structural parameters obtained from fitting of the EXAFS function of the catalysts after reduction at 773 K for 2 h and the amount of rhodium atoms atomically dispersed in the catalysts after reduction at 773 K for 2 h estimated from the STEM micrographs. See DOI: 10.1039/c4cp02596b b
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Rh–Cl bonds.5 The use of RhCl3nH2O and [RhCl(C2H4)2]2 also led to varied dispersion over sol–gel derived silica:6 RhCl3nH2O resulted in the formation of rhodium nanoparticles and crystallites with an irregular size while [RhCl(C2H4)2]2 resulted in nanoparticles with homogeneous distribution. The dispersion of platinum was reported to depend on both the metal precursor and on the supports of different nature, affecting in turn the performance of platinum catalysts in carbon monoxide oxidation.7 Alumina is widely used as support for catalysts2,8–10 and ceria is often added as a promoter due to its capability of stabilizing alumina11,12 and small metal particles via strong adhesion of the metal around oxygen vacancies.13 In our previous work we observed that ceria gets partially reduced under different atmospheres at a high temperature (773 K) and the addition of samaria as a promoter to the support further improves the reducibility of ceria.12 Additionally, ceria- and ceria-samaria-promoted catalysts conferred high stability to supported rhodium under oxidative, reductive and methane steam reforming (MSR) atmospheres.14,15 Force et al.16 studied ceria-supported rhodium catalysts prepared from two different rhodium precursors (rhodium(III) chloride and rhodium(III) nitrate) and found that the choice of precursor affects not only the dispersion, but also the reductive properties of the catalysts. Characterization techniques useful for attaining information on particle size and dispersion are scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS).17,18 Advances in STEM permit the imaging of species (Z-contrast) down to individual atoms.19 However, only small areas are analyzed in each micrograph. In extended X-ray absorption fine structure (EXAFS) analysis, every atom of an specific element is taken into account and the output is the average coordination around this element.20 The combination of these
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two characterization techniques makes it possible to precisely estimate metal size distribution and structure. Based on highresolution transmission electron microscopy (HRTEM) and EXAFS studies, Sun et al.21 illustrated the nature of dodecanethiolate palladium nanoparticles and the analysis of the discrepant results of the two techniques revealed a disordered phase composed of sulfurized palladium compounds. The effect of the metal precursor (rhodium(III) chloride or rhodium(I) acetate) and support type on the dispersion of rhodium after oxidative and reductive treatment was determined by means of scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS). The correlation of results obtained with these characterization techniques allowed the quantification of atomically dispersed rhodium in the samples.
Results and discussion Fig. 1 shows representative HAADF-STEM micrographs and the particle size distributions of (A) Rh/Al2O3_Cl and (B) Rh/12CeO2– Al2O3_Cl after reduction at 773 K for 2 h. Due to the atomic number (Z) contrast, the alumina support appears gray (ZAl = 13) while cerium (ZCe = 58) and rhodium (ZRh = 45) appear similarly bright in the micrographs. The Rh/Al2O3_Cl catalyst shows exclusively rhodium nanoparticles (blue circles) of size between
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0.3 and 1.7 nm (Fig. 1A). Rh/12CeO2–Al2O3_Cl has a broader size distribution showing nanoparticles (blue circles) of size between 0.3 and 2.3 nm and atomically dispersed rhodium (red circles). In addition clusters of a few atoms are observed (green circles). It is difficult to unambiguously assign the bright areas to rhodium or ceria, respectively, but the evaluation can be done as ceria commonly appears as large bright patches with distinguishable structure (Fig. S1 of the ESI†). Atomically dispersed species are not included in the particle size analysis thus impeding contributions from these species in the distribution. Fig. 2 shows representative STEM micrographs and particle size distribution of the (A) Rh/Al2O3_OAc, (B) Rh/12CeO2– Al2O3_OAc and (C) Rh/6Sm2O3–6CeO2–Al2O3_OAc catalysts after reduction at 773 K for 2 h. Atomically dispersed rhodium is observed in all samples but to a different extent. Rh/Al2O3_OAc shows mainly rhodium nanoparticles and clusters with sizes between 0.3 and 1.9 nm, mostly distributed at around 0.7 nm. Atomically dispersed rhodium is recognizable in one STEM micrograph of Rh/Al2O3_OAc Fig. 2A. Rh/12CeO2–Al2O3_OAc contains rhodium atomically dispersed, clusters and nanoparticles with a size of up to 1.7 nm. As previously reported15 samarium is highly dispersed in the 6Sm2O3–6CeO2–Al2O3 support after calcination at 773 K; however, after reduction at 873 K, samaria sinters strongly. Therefore, it is likely that the observed
Fig. 1 Representative HAADF-STEM micrographs and particle size distribution of (A) Rh/Al2O3_Cl and (B) Rh/12CeO2–Al2O3_Cl after reduction at 773 K for 2 h. Blue, green and red circles mark the nanoparticles, clusters and atomically dispersed rhodium, respectively.
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Fig. 2 Representative HAADF-STEM micrographs and particle size distribution of (A) Rh/Al2O3_OAc, (B) Rh/12CeO2–Al2O3_OAc and (C) Rh/6Sm2O3– 6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. Blue, green and red circles mark the nanoparticles, clusters and atomically dispersed rhodium, respectively.
atomically dispersed species in Fig. 2C are rhodium. Rhodium in the Rh/6Sm2O3–6CeO2–Al2O3_OAc is also present as nanoparticles with a size up to 1.5 nm and about 40% are 0.9 nm nanoparticles. The micrographs show no evidence of different particle shapes. Fig. 3 shows the Rh K-edge X-ray Absorption Near Edge Structure (XANES) spectra of (A) the references rhodium foil and rhodium oxide and in situ spectra of the catalysts (a) Rh/Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc (B) after calcination and (C) after reduction at 773 K for 2 h.
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The reference spectrum of Rh0 is dominated by two features at 23 230 and 23 260 eV while the spectrum of Rh3+ shows a more intense white line at 23 230 and a shoulder at 23 250 eV. The spectra of the different catalysts after calcination are rather similar and contain features of a fully oxidized state. After reduction at 773 K all spectra show a decrease in the intensity of the white line and appearance of a second feature pointing to rhodium reduction. Fig. 4 shows the experimental (solid line) and fitted (dashed line) (A) k2-weighted EXAFS oscillations and (B) Fourier transforms of the
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Fig. 3 Rh K-edge XANES spectra of (A) the references rhodium foil and rhodium oxide and in situ spectra of the catalysts: (a) Rh/Al2O3_Cl, (b) Rh/ 12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc (B) after calcination and (C) after reduction at 773 K for 2 h.
EXAFS functions of (a) Rh/Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3– 6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. The EXAFS oscillations of the rhodium K-edge of a reduced Rh/Al2O3 catalyst with highly dispersed rhodium species are the sum of oscillations due to Rh–Rh and Rh–O from support scattering.22 Accordingly, all samples show in the Fourier transformed EXAFS function a peak at E1.7 Å, assigned to the interference of Rh–Rh and Rh–O shells,23 and a peak at 2.5 Å, due to Rh–Rh contribution. Based on the work of Ogino et al.,24 which shows for zeolite supported rhodium complexes that the EXAFS spectra fitting is improved when adding a Rh–Al contribution, fitting models including a
Fig. 4 Experimental (solid line) and fitted (dashed line) (A) k2-weighted EXAFS oscillations and (B) Fourier transforms of the EXAFS functions of (a) Rh/ Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2– Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. Blue and red dashed lines indicate the Fourier transforms of the EXAFS functions of rhodium oxide and rhodium foil, respectively.
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Rh–Al shell were tested. Adding the Rh–Al contribution was possible; however it did not yield significant improvement of the quality of the fits. The intensity of the peaks varies for each sample and the Fourier transforms of the EXAFS function are dominated by the Rh–Rh backscattering pair, in accordance with the XANES spectra, which show features characteristic of the reduced state of rhodium. Fig. 5 shows the coordination numbers and interatomic distances for Rh–Rh and Rh–O shells obtained from EXAFS analysis of the Rh/xSm2O3–yCeO2–Al2O3 catalysts after reduction at 773 K for 2 h and cooling to room temperature in hydrogen. Table S1 of the ESI† shows the structural parameters obtained from the fitting of the first shell peaks. All samples show very small contributions of the Rh–O backscattering pair, indicating
Fig. 5 Coordination numbers (CN) and interatomic distances (R) for Rh–Rh and Rh–O shells obtained from EXAFS analysis of the first shell peaks of (a) Rh/ Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2– Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc after reduction at 773 K for 2 h.
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that rhodium interacts with the oxide supports. The measured Rh–O distance varies between 2.12 0.009 and 2.17 0.006 Å, which are values relatively different from the Rh–O distance for rhodium oxide (2.04 Å). The interaction of reduced rhodium with the oxygen atoms of the support may lead to larger bond lengths than observed in the oxide.22,25 The catalysts impregnated with the rhodium acetate precursor have a smaller averaged Rh–Rh CN than the respective catalysts impregnated with the rhodium chloride precursor: Rh/Al2O3_Cl and Rh/Al2O3_OAc have Rh–Rh CNs of 4.3 0.08 and 3.6 0.09, respectively; the Rh/12CeO2– Al2O3_Cl has a Rh–Rh CN of 3.8 0.05, while the Rh/12CeO2– Al2O3_OAc catalyst has a Rh–Rh CN of 3.1 0.09. The Rh/ 6Sm2O3–6CeO2–Al2O3_OAc sample has the lowest Rh–Rh CN with a value of 2.6 0.09. Corresponding statistical error bars are added; the errors associated with the Rh–O CN and interatomic distances are quite small (Table S1 of the ESI†). Frenkel et al.26 obtained similar Debye–Waller factors for platinum catalysts supported on carbon black with different platinum loadings. Fitting with a unique Debye–Waller factor value for the first Rh–Rh coordination shell of the different samples induces a decrease in the quality of the fits. The Rh–Rh distances vary between 2.63 0.002 Å for Rh/Al2O3_Cl and Rh/Al2O3_OAc and 2.68 0.001 Å for Rh/12CeO2–Al2O3_Cl, which is the same distance found for metallic rhodium. The combination of the data provided by the STEM and EXAFS measurements of the catalysts after reduction at 773 K for 2 h enables the determination of the fraction of rhodium present in nanoparticles as well as atomically dispersed using eqn (1). The number of atoms in each visible nanoparticle can be obtained from its size.27 Then the number of atoms can be correlated to the Rh–Rh CN according to Jentys28 and, taking into account the STEM particle size distribution, an average CN is obtained based on several STEM micrographs.29 As atomically dispersed species are not included in the particle size analysis done based on STEM, their contribution can be extracted from the CN obtained from the EXAFS analysis. The ratio of rhodium atoms atomically dispersed thus can be determined from: Nx x 100% CNTEM þ 100% CNatom ¼ CNEXAFS N N (1) N denotes the total number of rhodium atoms and x the number of atomically dispersed rhodium atoms, therefore (N x) corresponds to the amount of rhodium atoms present in nanoparticles. CNTEM is the Rh–Rh CN obtained from STEM micrographs as described above, CNatom corresponds to the Rh–Rh CN of atomically dispersed species (CNatom = 0) and CNEXAFS is the Rh–Rh CN obtained from the EXAFS analysis. Table 1 presents the Rh–Rh CN obtained from STEM, the CN obtained from the EXAFS analysis and the amount of atomically dispersed rhodium atoms calculated from eqn (1) in all catalysts after reduction at 773 K for 2 h. Rh/Al2O3_Cl shows very similar CNs obtained from STEM and EXAFS, equal to 4.4 and 4.3, respectively. The amount of atomically dispersed rhodium in this catalyst is negligible. The CNTEM obtained for
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Paper Table 1 Rh–Rh CN obtained from the analysis of the STEM micrographs, CN obtained from EXAFS and amount of atoms atomically dispersed (%) calculated from eqn (1) in the Rh/xSm2O3–yCeO2–Al2O3 catalysts after reduction at 773 K for 2 h
Sample Rh/Al2O3_Cl Rh/12CeO2–Al2O3_Cl Rh/Al2O3_OAc Rh/12CeO2–Al2O3_OAc Rh/6Sm2O3–6CeO2– Al2O3_OAc
Rh–Rh CNSTEM27,28
Rh–Rh CNEXAFS
Amount of rhodium atoms atomically dispersed (%)
4.4 5.4 4.2 4.8 5.8
4.3 3.8 3.6 3.1 2.6
2 30 15 36 55
(0.08) (0.05) (0.09) (0.09) (0.09)
Estimated overall systematic errors of the Rh–Rh CN obtained from fitting of the k2-weighted EXAFS function are given in parentheses.
Rh/12CeO2–Al2O3_Cl is 5.4, while the CNEXAFS is 3.8, implying that 30% of rhodium is atomically dispersed. Rh/Al2O3_OAc has a CNSTEM and a CNEXAFS of 4.2 and 3.6, respectively. This difference points to 15% of rhodium atomically dispersed. The CNSTEM and the CNEXAFS for the Rh/12CeO2–Al2O3_OAc catalyst is 4.8 and 3.1, respectively, which correspond to 36% of atomically dispersed rhodium. A CNTEM of 5.8 and a CNEXAFS of 2.6 were obtained for Rh/6Sm2O3–6CeO2–Al2O3_OAc resulting in the highest quantity of rhodium atomically dispersed, which amounts to 55%. The estimation of the amount of rhodium atoms atomically dispersed was also done by counting the individual atoms in a fixed area of the STEM micrographs and correlating to the number of atoms in nanoparticles27 in an equally large area (Table S2 of the ESI†). Similar values as the ones reported in Table 1 were obtained (8%); however, the quantitative analysis based on the STEM micrographs suggests roughly the same amount of rhodium atomically dispersed over Rh/12CeO2– Al2O3_Cl and Rh/12CeO2–Al2O3_OAc (34 and 33%, respectively). The choices of the rhodium precursor and of support are crucial to the catalyst design because they affect dispersion. The particle size distribution of the catalysts is broad, consisting of varied sizes from nanoparticles to atoms (Fig. 1 and 2). While STEM gives information on the particle size distribution, EXAFS gives the averaged coordination number of all rhodium atoms; the combined analysis of STEM and EXAFS gives detailed information on the structure of rhodium over the catalysts and allows the quantification of the amount of atomically dispersed rhodium (Table 1). The STEM micrographs show no evidence of different particle shapes, facilitating the comparison with the EXAFS results. Diverse shape distribution might lead to misleading comparison among techniques.30 Atomically dispersed atoms are mobile and tend to sinter under reaction conditions.15,31 The quantitative analysis of the amount of atomically dispersed rhodium points to the necessity of careful choice of metal precursor, tuned conditions for applied thermal treatments and designed supports for the attainment of relatively stable atomically dispersed rhodium catalysts. The data show a decrease in the amount of atomically dispersed rhodium in the order Rh/6Sm2O3–6CeO2–Al2O3_OAc 4 Rh/12CeO2–Al2O3_OAc 4 Rh/12CeO2–Al2O3_Cl 4 Rh/Al2O3_OAc 4 Rh/Al2O3_Cl. Quantitative analysis based exclusively on STEM micrographs (Table S2 of the
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ESI†) supports the accuracy of the results obtained from the combination of STEM and EXAFS. The individual rhodium atoms in the samaria-containing catalyst account for about half of the total amount of rhodium in the sample. Approximately two times more rhodium atoms are atomically dispersed over the ceria-promoted samples in comparison with the Rh/Al2O3_OAc. The stabilization effect conferred by ceria on the dispersion of rhodium is more pronounced than the effect of the use of rhodium acetate as a precursor in an unpromoted catalyst. Chen et al.4 performed scanning tunneling microscopy measurements to determine the size and the distribution of rhodium over Al2O3/NiAl when deposited from [Rh(OAc)2]2 and rhodium chloride; the decomposition of the precursors leads to the formation of dimers and large particles, respectively. Furthermore, monomeric rhodium species were formed by the decomposition of Rh(OAc)3 over alumina.32 During the decomposition of rhodium acetates the formation of a Rh–O bond with alumina preserves the number of the rhodium atoms in the core of the precursors.4 The origin of the nanoparticles formed from the chloride precursor is due to the strong Rh–Cl bonds.4,5 Decomposition of rhodium chloride under air occurs from 713 to 773 K.33 Decomposition of rhodium acetates is reported to be already complete at 650 K32 with further heating (up to 800 K) causing sintering of the highly dispersed rhodium species; rhodium dispersion decreases with increasing temperature of thermal treatment.34 Our previous work15 shows the formation of atomically dispersed rhodium after calcination at 773 K when samples are impregnated with the rhodium acetate precursor: the promoted catalysts have exclusively atomically dispersed rhodium and Rh/Al2O3 also contains small clusters. After reductive treatment at 873 K sintering becomes pronounced: Rh/Al2O3 shows the formation of nanoparticles and promoted supports maintain atomically dispersed rhodium to a reduced extent. Grunwaldt et al.25 observed a significant increase in the contribution of the Rh–Rh shell in Rh/Al2O3 catalysts after reduction at 773 K in comparison with a heating treatment under helium, indicating the increase in size of the metal particles under a reductive atmosphere. Alumina supported group-8 metals undergo aggregation into particles during treatment under hydrogen at high temperatures.31 As mentioned above, we previously showed15 that catalysts impregnated with rhodium acetate show improved stability of rhodium under different thermal treatments (under H2 up to 873 K and under MSR up to 1033 K) when promoted with ceria. The addition of samaria leads to enhanced stability. Similar results were obtained with catalysts synthesized with a rhodium chloride precursor:14 in situ X-ray absorption measurements carried out under MSR conditions showed that changes in the structure of the rhodium particles are less pronounced in the presence of both promoters. The XANES spectra of the catalysts after reduction at 773 K (Fig. 3) show features of rhodium in the reduced state; therefore, the Rh–O contribution in the Fourier transformed EXAFS function (Fig. 4 and 5) is due to the interaction of rhodium with the support.23,35 Van’t Blik et al.23 observed that during reduction rhodium crystallites are formed and attached to the support via Rh–O bonds. When under a reducing atmosphere at 773 K, ceria gets partially
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reduced and the addition of samaria increases the extent of ceria reduction.12 The stability of rhodium over the 6Sm2O3–6CeO2–Al2O3 support is enhanced, as can be seen from the higher amount of atomically dispersed rhodium (Table 1). Ceria as support favors high dispersion of metal particles and clusters, especially when promoted with lanthanides.14,15,36 The metal–support interaction is stronger around oxygen defects13 and such a mechanism partially explains metal resistance to sintering when supported on alumina promoted with reducible oxides. The addition of rare earth oxide promoters to alumina stabilizes its textural properties and prevents a decrease in surface area.12,14,37–41 The stabilization of alumina impacts in turn the dispersion of rhodium, inhibiting coalescence. A similar stabilization mechanism was reported for Pt/WOx-Al2O3 systems.42
Experimental Catalyst preparation Catalysts were prepared by wet impregnation over sol–gel14 or commercial g-alumina (Sigma-Aldrich). Sol–gel alumina was prepared by adding Al(OC4H9sec)3 to ethanol and water at 333 K. A solution of 0.11 mol L1 nitric acid was added to the mixture after 1 h. The gel was stirred for 14 h under reflux, dried at room temperature (RT) and then calcined at 773 K for 6 h. The ceria-promoted sol–gel alumina was obtained by wet impregnation at RT for 5 h with ethanol solution containing appropriate quantities of Ce(NO3)36H2O to obtain 12 wt%. Ethanol was removed using a rotary evaporator at 328 K. The promoted support was dried at 373 K for 12 h and calcined at 1223 K for 6 h. The bare or ceria-promoted sol–gel alumina supports were impregnated as described above with a solution of rhodium(III) chloride (RhCl3 20% in water) in ethanol and the samples were dried at 373 K overnight. Afterwards the catalysts were calcined at 873 K for 4 h. The catalysts obtained by impregnation with rhodium(III) chloride are referred to as Rh/Al2O3_Cl and Rh/12CeO2–Al2O3_Cl. Promoted commercial g-alumina supports were prepared by wet impregnation with solutions of Sm(NO3)36H2O and/or Ce(NO3)36H2O in ethanol as described above. The solids were dried at 373 K for 12 h and calcined at 773 K for 6 h. Rhodium impregnation was performed as described above with addition of ethanol solution containing a rhodium(I) acetate (RhOAc) precursor to the xSm2O3–yCeO2–Al2O3 supports, x and y being the theoretical concentrations of samaria and ceria, respectively, with value of 0, 6 or 12 wt%.15,32 The samples were dried at 373 K overnight and then calcined at 773 K for 4 h. These samples were abbreviated as Rh/xSm2O3–yCeO2–Al2O3_OAc. The amount of rhodium in the catalysts is about 0.5 wt%. as measured by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using a VISTA AX spectrometer (Varian, Agilent Technologies) and by Atomic Absorption Spectroscopy (AAS) using a Varian SpectrAA 220 FS spectrometer. STEM and XAS measurements High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements were performed
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using an aberration-corrected Hitachi HD-2700CS microscope, operated at 200 kV, producing images with atomic number (Z) contrast.19 A probe correction system (CEOS) incorporated in the microscope column between the condenser lens and the probe-forming objective lens enables a resolution of below 0.1 nm to be achieved.43 X-ray absorption spectroscopy (XAS) measurements at the Rh K-edge (E = 23 220 eV) were performed at the SuperXAS beamline at the Swiss Light Source (PSI, Villigen, Switzerland)44 using a Si(111) channel cut monochromator, a Pt-coated collimated mirror at 2.8 mrad and a Pt-coated toroidal focusing mirror. The measurements were done in transmission mode in a 0.8 mm capillary reactor with a wall thickness of 0.01 mm heated by a gas blower oven and enclosing about 10 mg of catalyst. Spectra were acquired in transmission mode using an ionization chamber detector in the energy range from 23 120 to 24 200 eV. Data in the XANES and EXAFS regions were collected with step sizes of 1.4 and 3 eV, respectively. A mass spectrometer (HIDEN Analytical) was plugged to the end of the reactor for analysis of the outlet gases. The catalysts were heated at 5 K min1 under 20 mL min1 of 20% O2/He up to 773 K for 2 h. Sequentially the samples were cooled down, reduced under 20 mL min1 of 10% H2/He also at 5 K min1 up to 773 K for 2 h and cooled to room temperature in hydrogen. XAS spectra were collected after the oxidative and reductive treatments. The EXAFS analysis of the spectra was done by means of XDAP45 and data reduction was done using standard procedures.46 Reference XAS files were previously generated from the known parameters of the unit cell of the Rh metal and Rh2O3 using atoms,47 calculated using the FEFF8 code48 and fitted to the measured data of rhodium foil and rhodium oxide. Ogino et al. provided us with the reference XAS file of the Rh–Al scatter pair.24,49 Fitting of the EXAFS spectra was done in R-space (0.5 o R o 4.0 Å) with k2-weighting and k ranging from 2.5 to 10.0 Å1.
Conclusions The complementary analysis of STEM and EXAFS results allowed the determination of the amount of atomically dispersed rhodium over catalysts. The nature of the support has a large influence on the sintering of rhodium and the highest amount of atomically dispersed rhodium is obtained when samaria is added to a ceria-containing catalyst impregnated with a rhodium acetate precursor. The use of rhodium acetate as a precursor results in a higher amount of atomically dispersed rhodium when compared with the respective samples synthesized with rhodium chloride. The addition of ceria to the support increases the dispersion of rhodium, independent of the metal precursor used. Finely tuned thermal treatments, careful choice of metal precursor and support type are conditions for the synthesis of stable atomically dispersed catalysts.
Acknowledgements The authors are grateful for the beamtime provided at the SuperXAS beamline, in the Swiss Light Source at the Paul
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Scherrer Institute, Villigen, Switzerland. We thank Urs Hartfelder, ´ Kopelent and Mattha ¨us Rothensteiner for Cristina Paun, Rene assistance with the XAS measurements. We also thank Bruce C. Gates, Cong-Yan Chen, Isao Ogino and Joseph D. Kistler for the provision of the crystallographic parameters and reference file of the Rh–Al scatter pair. The electron microscopy center of ETH Zurich (EMEZ) is acknowledged for providing measurement time.
Notes and references 1 R. A. Van Santen, Acc. Chem. Res., 2009, 42, 57. 2 J. A. Schwarz, C. Contescu and A. Contescu, Chem. Rev., 1995, 95, 477. 3 E. S. Vasiliadou, E. Heracleous, I. A. Vasalos and A. A. Lemonidou, Appl. Catal., B, 2009, 92, 90. 4 Z. W. Chen, S. Fujita and K. Fukui, J. Phys. Chem. C, 2011, 115, 14270. 5 H. Barnighausen and B. K. Handa, J. Less-Common Met., 1964, 6, 226. 6 R. Campostrini, M. Ischia, G. Carturan, S. Gialanella and L. Armelao, J. Sol-Gel Sci. Technol., 2000, 18, 61. 7 Z. Jiang, Y. Yang, W. F. Shangguan and Z. Jiang, J. Phys. Chem. C, 2012, 116, 19396. 8 H. Pines and W. O. Haag, J. Am. Chem. Soc., 1960, 82, 2471. 9 D. L. Trimm and A. Stanislaus, Appl. Catal., 1986, 21, 215. 10 M. Trueba and S. P. Trasatti, Eur. J. Inorg. Chem., 2005, 3393. 11 S. Damyanova, C. A. Perez, M. Schmal and J. M. C. Bueno, Appl. Catal., A, 2002, 234, 271. 12 R. B. Duarte, O. V. Safonova, F. Krumeich, M. Makosch and J. A. Van Bokhoven, ACS Catal., 2013, 3, 1956. 13 J. A. Farmer and C. T. Campbell, Science, 2010, 329, 933. 14 R. B. Duarte, M. Nachtegaal, J. M. C. Bueno and J. A. van Bokhoven, J. Catal., 2012, 296, 86. 15 R. B. Duarte, F. Krumeich and J. A. van Bokhoven, ACS Catal., 2014, 4, 1279. 16 C. Force, J. P. Belzunegui, J. Sanz, A. Martinez-Arias and J. Soria, J. Catal., 2001, 197, 192. 17 F. Krumeich, S. Marx, A. Baiker and R. Nesper, Z. Anorg. Allg. Chem., 2011, 637, 875. 18 S. Marx, F. Krumeich and A. Baiker, J. Phys. Chem. C, 2011, 115, 8195. 19 S. J. Pennycook, Ultramicroscopy, 2012, 123, 28. 20 S. Bordiga, E. Groppo, G. Agostini, J. A. van Bokhoven and C. Lamberti, Chem. Rev., 2013, 113, 1736. 21 Y. Sun, A. I. Frenkel, R. Isseroff, C. Shonbrun, M. Forman, K. W. Shin, T. Koga, H. White, L. H. Zhang, Y. M. Zhu, M. H. Rafailovich and J. C. Sokolov, Langmuir, 2006, 22, 807. 22 J. B. A. D. Vanzon, D. C. Koningsberger, H. F. J. Vantblik, R. Prins and D. E. Sayers, J. Chem. Phys., 1984, 80, 3914. 23 H. F. J. Van’t Blik, J. B. A. D. Vanzon, D. C. Koningsberger and R. Prins, J. Mol. Catal., 1984, 25, 379. 24 I. Ogino, C. Y. Chen and B. C. Gates, Dalton Trans., 2010, 39, 8423. 25 J. D. Grunwaldt, L. Basini and B. S. Clausen, J. Catal., 2001, 200, 321.
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26 A. I. Frenkel, C. W. Hills and R. G. Nuzzo, J. Phys. Chem. B, 2001, 105, 12689. 27 J. de Graaf, A. J. van Dillen, K. P. de Jong and D. C. Koningsberger, J. Catal., 2001, 203, 307. 28 A. Jentys, Phys. Chem. Chem. Phys., 1999, 1, 4059. 29 G. Agostini, R. Pellegrini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A. Zecchina and C. Lamberti, J. Phys. Chem. C, 2009, 113, 10485. 30 G. Agostini, A. Piovano, L. Bertinetti, R. Pellegrini, G. Leofanti, E. Groppo and C. Lamberti, J. Phys. Chem. C, 2014, 118, 4085. 31 A. Uzun, V. Ortalan, N. D. Browning and B. C. Gates, J. Catal., 2010, 269, 318. 32 Z. Chen, W. Chun and K. Fukui, Photon Factory Activity Report, 2010, 27, 54. 33 Y. I. Ivashentsev and R. I. Timonova, Zh. Neorg. Khim., 1966, 11, 2189. 34 S. E. Wanke, in Sintering and Heterogeneous Catalysis, ed. G. C. Kuczynski, A. E. Miller and G. A. Sargent, Plenum Press, New York, 1983, p. 223. 35 D. C. Koningsberger, J. H. A. Martens, R. Prins, D. R. Short and D. E. Sayers, J. Phys. Chem., 1986, 90, 3047. 36 L. Kundakovic and M. Flytzani-Stephanopoulos, J. Catal., 1998, 179, 203. 37 M. Ozawa, M. Kimura and A. Isogai, J. Less-Common Met., 1990, 162, 297.
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38 I. I. M. Tijburg, J. W. Geus and H. W. Zandbergen, J. Mater. Sci., 1991, 26, 6479. 39 G. C. Vanleerdam, H. H. Brongersma, I. I. M. Tijburg and J. W. Geus, Appl. Surf. Sci., 1992, 55, 11. 40 L. Wachowski, P. Kirszensztejn, R. Lopatka and B. Czajka, Mater. Chem. Phys., 1994, 37, 29. 41 M. Ozawa, J. Alloys Compd., 2006, 408, 1090. 42 J. L. Contreras and G. A. Fuentes, 11th International Congress on Catalysis - 40th Anniversary, Pts a and B, 1996, 101, 1195. 43 H. Inada, L. J. Wu, J. Wall, D. Su and Y. M. Zhu, J. Electron Microsc., 2009, 58, 111. 44 P. M. Abdala, O. V. Safonova, G. Wiker, W. van Beek, H. Emerich, J. A. van Bokhoven, J. Sa, J. Szlachetko and M. Nachtegaal, Chimia, 2012, 66, 699. 45 M. Vaarkamp, J. C. Linders and D. C. Koningsberger, Physica B, 1995, 208, 159. 46 J. A. van Bokhoven, T. Ressler, F. M. F. de Groot and G. Knopp-Gericke, In-Situ Spectroscopy of Catalysts, California, 2004. 47 B. Ravel, J. Synchrotron Radiat., 2001, 8, 314. 48 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7565. 49 W. B. Pearson, L. D. Calvert and P. Villars, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, American Society for Metals, Metals Park, OH, 1985.
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Atomically dispersed rhodium on a support: the influence of a metal precursor and a support† R. B Duarte,a O. V. Safonova,b F. Krumeicha and J. A. van Bokhoven*ab The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation
Received 12th June 2014, Accepted 15th July 2014
is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over
DOI: 10.1039/c4cp02596b
the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate
supports of the same composition. The stability of rhodium is improved with the addition of promoters; precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive
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treatment at 773 K.
Introduction Numerous catalytic reactions are structure sensitive1 and therefore the precise control of metal dispersion is of ultimate importance. Stable supported atomically dispersed catalysts are highly desirable for heterogeneous catalytic reactions in which s-bond activation is kinetically relevant. A common method for the synthesis of supported metal catalysts is wet impregnation of the support with a metal precursor solution.2 Metal dispersion and properties of catalysts are influenced by the type of support and the nature of the metal precursor: Vasiliadou et al.3 observed higher activity of ruthenium-based catalysts synthesized with RuCl3xH2O in comparison with RuNO(NO3)3 in the hydrogenolysis of biomass due to the retention of Cl ions on the support surface. The dispersion of ruthenium was similar to and independent of the precursor type; only the nature of the oxidic support (alumina, silica and zirconia) was found to influence the dispersion of the metal. In contrast, thermal decomposition of different rhodium precursors (rhodium(II) acetate and rhodium(III) chloride) was reported to yield varied dispersion when supported on alumina.4 Rhodium(II) acetate led to higher dispersion due to the interaction of the metal with oxygen from the support and the use of rhodium(III) chloride favored sintering because of the strength of a
ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. E-mail: [email protected] † Electronic supplementary information (ESI) available: A HAADF-STEM image of a CeO2 crystal supported on the alumina, structural parameters obtained from fitting of the EXAFS function of the catalysts after reduction at 773 K for 2 h and the amount of rhodium atoms atomically dispersed in the catalysts after reduction at 773 K for 2 h estimated from the STEM micrographs. See DOI: 10.1039/c4cp02596b b
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Rh–Cl bonds.5 The use of RhCl3nH2O and [RhCl(C2H4)2]2 also led to varied dispersion over sol–gel derived silica:6 RhCl3nH2O resulted in the formation of rhodium nanoparticles and crystallites with an irregular size while [RhCl(C2H4)2]2 resulted in nanoparticles with homogeneous distribution. The dispersion of platinum was reported to depend on both the metal precursor and on the supports of different nature, affecting in turn the performance of platinum catalysts in carbon monoxide oxidation.7 Alumina is widely used as support for catalysts2,8–10 and ceria is often added as a promoter due to its capability of stabilizing alumina11,12 and small metal particles via strong adhesion of the metal around oxygen vacancies.13 In our previous work we observed that ceria gets partially reduced under different atmospheres at a high temperature (773 K) and the addition of samaria as a promoter to the support further improves the reducibility of ceria.12 Additionally, ceria- and ceria-samaria-promoted catalysts conferred high stability to supported rhodium under oxidative, reductive and methane steam reforming (MSR) atmospheres.14,15 Force et al.16 studied ceria-supported rhodium catalysts prepared from two different rhodium precursors (rhodium(III) chloride and rhodium(III) nitrate) and found that the choice of precursor affects not only the dispersion, but also the reductive properties of the catalysts. Characterization techniques useful for attaining information on particle size and dispersion are scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS).17,18 Advances in STEM permit the imaging of species (Z-contrast) down to individual atoms.19 However, only small areas are analyzed in each micrograph. In extended X-ray absorption fine structure (EXAFS) analysis, every atom of an specific element is taken into account and the output is the average coordination around this element.20 The combination of these
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two characterization techniques makes it possible to precisely estimate metal size distribution and structure. Based on highresolution transmission electron microscopy (HRTEM) and EXAFS studies, Sun et al.21 illustrated the nature of dodecanethiolate palladium nanoparticles and the analysis of the discrepant results of the two techniques revealed a disordered phase composed of sulfurized palladium compounds. The effect of the metal precursor (rhodium(III) chloride or rhodium(I) acetate) and support type on the dispersion of rhodium after oxidative and reductive treatment was determined by means of scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS). The correlation of results obtained with these characterization techniques allowed the quantification of atomically dispersed rhodium in the samples.
Results and discussion Fig. 1 shows representative HAADF-STEM micrographs and the particle size distributions of (A) Rh/Al2O3_Cl and (B) Rh/12CeO2– Al2O3_Cl after reduction at 773 K for 2 h. Due to the atomic number (Z) contrast, the alumina support appears gray (ZAl = 13) while cerium (ZCe = 58) and rhodium (ZRh = 45) appear similarly bright in the micrographs. The Rh/Al2O3_Cl catalyst shows exclusively rhodium nanoparticles (blue circles) of size between
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0.3 and 1.7 nm (Fig. 1A). Rh/12CeO2–Al2O3_Cl has a broader size distribution showing nanoparticles (blue circles) of size between 0.3 and 2.3 nm and atomically dispersed rhodium (red circles). In addition clusters of a few atoms are observed (green circles). It is difficult to unambiguously assign the bright areas to rhodium or ceria, respectively, but the evaluation can be done as ceria commonly appears as large bright patches with distinguishable structure (Fig. S1 of the ESI†). Atomically dispersed species are not included in the particle size analysis thus impeding contributions from these species in the distribution. Fig. 2 shows representative STEM micrographs and particle size distribution of the (A) Rh/Al2O3_OAc, (B) Rh/12CeO2– Al2O3_OAc and (C) Rh/6Sm2O3–6CeO2–Al2O3_OAc catalysts after reduction at 773 K for 2 h. Atomically dispersed rhodium is observed in all samples but to a different extent. Rh/Al2O3_OAc shows mainly rhodium nanoparticles and clusters with sizes between 0.3 and 1.9 nm, mostly distributed at around 0.7 nm. Atomically dispersed rhodium is recognizable in one STEM micrograph of Rh/Al2O3_OAc Fig. 2A. Rh/12CeO2–Al2O3_OAc contains rhodium atomically dispersed, clusters and nanoparticles with a size of up to 1.7 nm. As previously reported15 samarium is highly dispersed in the 6Sm2O3–6CeO2–Al2O3 support after calcination at 773 K; however, after reduction at 873 K, samaria sinters strongly. Therefore, it is likely that the observed
Fig. 1 Representative HAADF-STEM micrographs and particle size distribution of (A) Rh/Al2O3_Cl and (B) Rh/12CeO2–Al2O3_Cl after reduction at 773 K for 2 h. Blue, green and red circles mark the nanoparticles, clusters and atomically dispersed rhodium, respectively.
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Fig. 2 Representative HAADF-STEM micrographs and particle size distribution of (A) Rh/Al2O3_OAc, (B) Rh/12CeO2–Al2O3_OAc and (C) Rh/6Sm2O3– 6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. Blue, green and red circles mark the nanoparticles, clusters and atomically dispersed rhodium, respectively.
atomically dispersed species in Fig. 2C are rhodium. Rhodium in the Rh/6Sm2O3–6CeO2–Al2O3_OAc is also present as nanoparticles with a size up to 1.5 nm and about 40% are 0.9 nm nanoparticles. The micrographs show no evidence of different particle shapes. Fig. 3 shows the Rh K-edge X-ray Absorption Near Edge Structure (XANES) spectra of (A) the references rhodium foil and rhodium oxide and in situ spectra of the catalysts (a) Rh/Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc (B) after calcination and (C) after reduction at 773 K for 2 h.
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The reference spectrum of Rh0 is dominated by two features at 23 230 and 23 260 eV while the spectrum of Rh3+ shows a more intense white line at 23 230 and a shoulder at 23 250 eV. The spectra of the different catalysts after calcination are rather similar and contain features of a fully oxidized state. After reduction at 773 K all spectra show a decrease in the intensity of the white line and appearance of a second feature pointing to rhodium reduction. Fig. 4 shows the experimental (solid line) and fitted (dashed line) (A) k2-weighted EXAFS oscillations and (B) Fourier transforms of the
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Fig. 3 Rh K-edge XANES spectra of (A) the references rhodium foil and rhodium oxide and in situ spectra of the catalysts: (a) Rh/Al2O3_Cl, (b) Rh/ 12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc (B) after calcination and (C) after reduction at 773 K for 2 h.
EXAFS functions of (a) Rh/Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2–Al2O3_OAc and (e) Rh/6Sm2O3– 6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. The EXAFS oscillations of the rhodium K-edge of a reduced Rh/Al2O3 catalyst with highly dispersed rhodium species are the sum of oscillations due to Rh–Rh and Rh–O from support scattering.22 Accordingly, all samples show in the Fourier transformed EXAFS function a peak at E1.7 Å, assigned to the interference of Rh–Rh and Rh–O shells,23 and a peak at 2.5 Å, due to Rh–Rh contribution. Based on the work of Ogino et al.,24 which shows for zeolite supported rhodium complexes that the EXAFS spectra fitting is improved when adding a Rh–Al contribution, fitting models including a
Fig. 4 Experimental (solid line) and fitted (dashed line) (A) k2-weighted EXAFS oscillations and (B) Fourier transforms of the EXAFS functions of (a) Rh/ Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2– Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc after reduction at 773 K for 2 h. Blue and red dashed lines indicate the Fourier transforms of the EXAFS functions of rhodium oxide and rhodium foil, respectively.
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Rh–Al shell were tested. Adding the Rh–Al contribution was possible; however it did not yield significant improvement of the quality of the fits. The intensity of the peaks varies for each sample and the Fourier transforms of the EXAFS function are dominated by the Rh–Rh backscattering pair, in accordance with the XANES spectra, which show features characteristic of the reduced state of rhodium. Fig. 5 shows the coordination numbers and interatomic distances for Rh–Rh and Rh–O shells obtained from EXAFS analysis of the Rh/xSm2O3–yCeO2–Al2O3 catalysts after reduction at 773 K for 2 h and cooling to room temperature in hydrogen. Table S1 of the ESI† shows the structural parameters obtained from the fitting of the first shell peaks. All samples show very small contributions of the Rh–O backscattering pair, indicating
Fig. 5 Coordination numbers (CN) and interatomic distances (R) for Rh–Rh and Rh–O shells obtained from EXAFS analysis of the first shell peaks of (a) Rh/ Al2O3_Cl, (b) Rh/12CeO2–Al2O3_Cl, (c) Rh/Al2O3_OAc, (d) Rh/12CeO2– Al2O3_OAc and (e) Rh/6Sm2O3–6CeO2–Al2O3_OAc after reduction at 773 K for 2 h.
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that rhodium interacts with the oxide supports. The measured Rh–O distance varies between 2.12 0.009 and 2.17 0.006 Å, which are values relatively different from the Rh–O distance for rhodium oxide (2.04 Å). The interaction of reduced rhodium with the oxygen atoms of the support may lead to larger bond lengths than observed in the oxide.22,25 The catalysts impregnated with the rhodium acetate precursor have a smaller averaged Rh–Rh CN than the respective catalysts impregnated with the rhodium chloride precursor: Rh/Al2O3_Cl and Rh/Al2O3_OAc have Rh–Rh CNs of 4.3 0.08 and 3.6 0.09, respectively; the Rh/12CeO2– Al2O3_Cl has a Rh–Rh CN of 3.8 0.05, while the Rh/12CeO2– Al2O3_OAc catalyst has a Rh–Rh CN of 3.1 0.09. The Rh/ 6Sm2O3–6CeO2–Al2O3_OAc sample has the lowest Rh–Rh CN with a value of 2.6 0.09. Corresponding statistical error bars are added; the errors associated with the Rh–O CN and interatomic distances are quite small (Table S1 of the ESI†). Frenkel et al.26 obtained similar Debye–Waller factors for platinum catalysts supported on carbon black with different platinum loadings. Fitting with a unique Debye–Waller factor value for the first Rh–Rh coordination shell of the different samples induces a decrease in the quality of the fits. The Rh–Rh distances vary between 2.63 0.002 Å for Rh/Al2O3_Cl and Rh/Al2O3_OAc and 2.68 0.001 Å for Rh/12CeO2–Al2O3_Cl, which is the same distance found for metallic rhodium. The combination of the data provided by the STEM and EXAFS measurements of the catalysts after reduction at 773 K for 2 h enables the determination of the fraction of rhodium present in nanoparticles as well as atomically dispersed using eqn (1). The number of atoms in each visible nanoparticle can be obtained from its size.27 Then the number of atoms can be correlated to the Rh–Rh CN according to Jentys28 and, taking into account the STEM particle size distribution, an average CN is obtained based on several STEM micrographs.29 As atomically dispersed species are not included in the particle size analysis done based on STEM, their contribution can be extracted from the CN obtained from the EXAFS analysis. The ratio of rhodium atoms atomically dispersed thus can be determined from: Nx x 100% CNTEM þ 100% CNatom ¼ CNEXAFS N N (1) N denotes the total number of rhodium atoms and x the number of atomically dispersed rhodium atoms, therefore (N x) corresponds to the amount of rhodium atoms present in nanoparticles. CNTEM is the Rh–Rh CN obtained from STEM micrographs as described above, CNatom corresponds to the Rh–Rh CN of atomically dispersed species (CNatom = 0) and CNEXAFS is the Rh–Rh CN obtained from the EXAFS analysis. Table 1 presents the Rh–Rh CN obtained from STEM, the CN obtained from the EXAFS analysis and the amount of atomically dispersed rhodium atoms calculated from eqn (1) in all catalysts after reduction at 773 K for 2 h. Rh/Al2O3_Cl shows very similar CNs obtained from STEM and EXAFS, equal to 4.4 and 4.3, respectively. The amount of atomically dispersed rhodium in this catalyst is negligible. The CNTEM obtained for
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Paper Table 1 Rh–Rh CN obtained from the analysis of the STEM micrographs, CN obtained from EXAFS and amount of atoms atomically dispersed (%) calculated from eqn (1) in the Rh/xSm2O3–yCeO2–Al2O3 catalysts after reduction at 773 K for 2 h
Sample Rh/Al2O3_Cl Rh/12CeO2–Al2O3_Cl Rh/Al2O3_OAc Rh/12CeO2–Al2O3_OAc Rh/6Sm2O3–6CeO2– Al2O3_OAc
Rh–Rh CNSTEM27,28
Rh–Rh CNEXAFS
Amount of rhodium atoms atomically dispersed (%)
4.4 5.4 4.2 4.8 5.8
4.3 3.8 3.6 3.1 2.6
2 30 15 36 55
(0.08) (0.05) (0.09) (0.09) (0.09)
Estimated overall systematic errors of the Rh–Rh CN obtained from fitting of the k2-weighted EXAFS function are given in parentheses.
Rh/12CeO2–Al2O3_Cl is 5.4, while the CNEXAFS is 3.8, implying that 30% of rhodium is atomically dispersed. Rh/Al2O3_OAc has a CNSTEM and a CNEXAFS of 4.2 and 3.6, respectively. This difference points to 15% of rhodium atomically dispersed. The CNSTEM and the CNEXAFS for the Rh/12CeO2–Al2O3_OAc catalyst is 4.8 and 3.1, respectively, which correspond to 36% of atomically dispersed rhodium. A CNTEM of 5.8 and a CNEXAFS of 2.6 were obtained for Rh/6Sm2O3–6CeO2–Al2O3_OAc resulting in the highest quantity of rhodium atomically dispersed, which amounts to 55%. The estimation of the amount of rhodium atoms atomically dispersed was also done by counting the individual atoms in a fixed area of the STEM micrographs and correlating to the number of atoms in nanoparticles27 in an equally large area (Table S2 of the ESI†). Similar values as the ones reported in Table 1 were obtained (8%); however, the quantitative analysis based on the STEM micrographs suggests roughly the same amount of rhodium atomically dispersed over Rh/12CeO2– Al2O3_Cl and Rh/12CeO2–Al2O3_OAc (34 and 33%, respectively). The choices of the rhodium precursor and of support are crucial to the catalyst design because they affect dispersion. The particle size distribution of the catalysts is broad, consisting of varied sizes from nanoparticles to atoms (Fig. 1 and 2). While STEM gives information on the particle size distribution, EXAFS gives the averaged coordination number of all rhodium atoms; the combined analysis of STEM and EXAFS gives detailed information on the structure of rhodium over the catalysts and allows the quantification of the amount of atomically dispersed rhodium (Table 1). The STEM micrographs show no evidence of different particle shapes, facilitating the comparison with the EXAFS results. Diverse shape distribution might lead to misleading comparison among techniques.30 Atomically dispersed atoms are mobile and tend to sinter under reaction conditions.15,31 The quantitative analysis of the amount of atomically dispersed rhodium points to the necessity of careful choice of metal precursor, tuned conditions for applied thermal treatments and designed supports for the attainment of relatively stable atomically dispersed rhodium catalysts. The data show a decrease in the amount of atomically dispersed rhodium in the order Rh/6Sm2O3–6CeO2–Al2O3_OAc 4 Rh/12CeO2–Al2O3_OAc 4 Rh/12CeO2–Al2O3_Cl 4 Rh/Al2O3_OAc 4 Rh/Al2O3_Cl. Quantitative analysis based exclusively on STEM micrographs (Table S2 of the
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ESI†) supports the accuracy of the results obtained from the combination of STEM and EXAFS. The individual rhodium atoms in the samaria-containing catalyst account for about half of the total amount of rhodium in the sample. Approximately two times more rhodium atoms are atomically dispersed over the ceria-promoted samples in comparison with the Rh/Al2O3_OAc. The stabilization effect conferred by ceria on the dispersion of rhodium is more pronounced than the effect of the use of rhodium acetate as a precursor in an unpromoted catalyst. Chen et al.4 performed scanning tunneling microscopy measurements to determine the size and the distribution of rhodium over Al2O3/NiAl when deposited from [Rh(OAc)2]2 and rhodium chloride; the decomposition of the precursors leads to the formation of dimers and large particles, respectively. Furthermore, monomeric rhodium species were formed by the decomposition of Rh(OAc)3 over alumina.32 During the decomposition of rhodium acetates the formation of a Rh–O bond with alumina preserves the number of the rhodium atoms in the core of the precursors.4 The origin of the nanoparticles formed from the chloride precursor is due to the strong Rh–Cl bonds.4,5 Decomposition of rhodium chloride under air occurs from 713 to 773 K.33 Decomposition of rhodium acetates is reported to be already complete at 650 K32 with further heating (up to 800 K) causing sintering of the highly dispersed rhodium species; rhodium dispersion decreases with increasing temperature of thermal treatment.34 Our previous work15 shows the formation of atomically dispersed rhodium after calcination at 773 K when samples are impregnated with the rhodium acetate precursor: the promoted catalysts have exclusively atomically dispersed rhodium and Rh/Al2O3 also contains small clusters. After reductive treatment at 873 K sintering becomes pronounced: Rh/Al2O3 shows the formation of nanoparticles and promoted supports maintain atomically dispersed rhodium to a reduced extent. Grunwaldt et al.25 observed a significant increase in the contribution of the Rh–Rh shell in Rh/Al2O3 catalysts after reduction at 773 K in comparison with a heating treatment under helium, indicating the increase in size of the metal particles under a reductive atmosphere. Alumina supported group-8 metals undergo aggregation into particles during treatment under hydrogen at high temperatures.31 As mentioned above, we previously showed15 that catalysts impregnated with rhodium acetate show improved stability of rhodium under different thermal treatments (under H2 up to 873 K and under MSR up to 1033 K) when promoted with ceria. The addition of samaria leads to enhanced stability. Similar results were obtained with catalysts synthesized with a rhodium chloride precursor:14 in situ X-ray absorption measurements carried out under MSR conditions showed that changes in the structure of the rhodium particles are less pronounced in the presence of both promoters. The XANES spectra of the catalysts after reduction at 773 K (Fig. 3) show features of rhodium in the reduced state; therefore, the Rh–O contribution in the Fourier transformed EXAFS function (Fig. 4 and 5) is due to the interaction of rhodium with the support.23,35 Van’t Blik et al.23 observed that during reduction rhodium crystallites are formed and attached to the support via Rh–O bonds. When under a reducing atmosphere at 773 K, ceria gets partially
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reduced and the addition of samaria increases the extent of ceria reduction.12 The stability of rhodium over the 6Sm2O3–6CeO2–Al2O3 support is enhanced, as can be seen from the higher amount of atomically dispersed rhodium (Table 1). Ceria as support favors high dispersion of metal particles and clusters, especially when promoted with lanthanides.14,15,36 The metal–support interaction is stronger around oxygen defects13 and such a mechanism partially explains metal resistance to sintering when supported on alumina promoted with reducible oxides. The addition of rare earth oxide promoters to alumina stabilizes its textural properties and prevents a decrease in surface area.12,14,37–41 The stabilization of alumina impacts in turn the dispersion of rhodium, inhibiting coalescence. A similar stabilization mechanism was reported for Pt/WOx-Al2O3 systems.42
Experimental Catalyst preparation Catalysts were prepared by wet impregnation over sol–gel14 or commercial g-alumina (Sigma-Aldrich). Sol–gel alumina was prepared by adding Al(OC4H9sec)3 to ethanol and water at 333 K. A solution of 0.11 mol L1 nitric acid was added to the mixture after 1 h. The gel was stirred for 14 h under reflux, dried at room temperature (RT) and then calcined at 773 K for 6 h. The ceria-promoted sol–gel alumina was obtained by wet impregnation at RT for 5 h with ethanol solution containing appropriate quantities of Ce(NO3)36H2O to obtain 12 wt%. Ethanol was removed using a rotary evaporator at 328 K. The promoted support was dried at 373 K for 12 h and calcined at 1223 K for 6 h. The bare or ceria-promoted sol–gel alumina supports were impregnated as described above with a solution of rhodium(III) chloride (RhCl3 20% in water) in ethanol and the samples were dried at 373 K overnight. Afterwards the catalysts were calcined at 873 K for 4 h. The catalysts obtained by impregnation with rhodium(III) chloride are referred to as Rh/Al2O3_Cl and Rh/12CeO2–Al2O3_Cl. Promoted commercial g-alumina supports were prepared by wet impregnation with solutions of Sm(NO3)36H2O and/or Ce(NO3)36H2O in ethanol as described above. The solids were dried at 373 K for 12 h and calcined at 773 K for 6 h. Rhodium impregnation was performed as described above with addition of ethanol solution containing a rhodium(I) acetate (RhOAc) precursor to the xSm2O3–yCeO2–Al2O3 supports, x and y being the theoretical concentrations of samaria and ceria, respectively, with value of 0, 6 or 12 wt%.15,32 The samples were dried at 373 K overnight and then calcined at 773 K for 4 h. These samples were abbreviated as Rh/xSm2O3–yCeO2–Al2O3_OAc. The amount of rhodium in the catalysts is about 0.5 wt%. as measured by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using a VISTA AX spectrometer (Varian, Agilent Technologies) and by Atomic Absorption Spectroscopy (AAS) using a Varian SpectrAA 220 FS spectrometer. STEM and XAS measurements High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements were performed
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using an aberration-corrected Hitachi HD-2700CS microscope, operated at 200 kV, producing images with atomic number (Z) contrast.19 A probe correction system (CEOS) incorporated in the microscope column between the condenser lens and the probe-forming objective lens enables a resolution of below 0.1 nm to be achieved.43 X-ray absorption spectroscopy (XAS) measurements at the Rh K-edge (E = 23 220 eV) were performed at the SuperXAS beamline at the Swiss Light Source (PSI, Villigen, Switzerland)44 using a Si(111) channel cut monochromator, a Pt-coated collimated mirror at 2.8 mrad and a Pt-coated toroidal focusing mirror. The measurements were done in transmission mode in a 0.8 mm capillary reactor with a wall thickness of 0.01 mm heated by a gas blower oven and enclosing about 10 mg of catalyst. Spectra were acquired in transmission mode using an ionization chamber detector in the energy range from 23 120 to 24 200 eV. Data in the XANES and EXAFS regions were collected with step sizes of 1.4 and 3 eV, respectively. A mass spectrometer (HIDEN Analytical) was plugged to the end of the reactor for analysis of the outlet gases. The catalysts were heated at 5 K min1 under 20 mL min1 of 20% O2/He up to 773 K for 2 h. Sequentially the samples were cooled down, reduced under 20 mL min1 of 10% H2/He also at 5 K min1 up to 773 K for 2 h and cooled to room temperature in hydrogen. XAS spectra were collected after the oxidative and reductive treatments. The EXAFS analysis of the spectra was done by means of XDAP45 and data reduction was done using standard procedures.46 Reference XAS files were previously generated from the known parameters of the unit cell of the Rh metal and Rh2O3 using atoms,47 calculated using the FEFF8 code48 and fitted to the measured data of rhodium foil and rhodium oxide. Ogino et al. provided us with the reference XAS file of the Rh–Al scatter pair.24,49 Fitting of the EXAFS spectra was done in R-space (0.5 o R o 4.0 Å) with k2-weighting and k ranging from 2.5 to 10.0 Å1.
Conclusions The complementary analysis of STEM and EXAFS results allowed the determination of the amount of atomically dispersed rhodium over catalysts. The nature of the support has a large influence on the sintering of rhodium and the highest amount of atomically dispersed rhodium is obtained when samaria is added to a ceria-containing catalyst impregnated with a rhodium acetate precursor. The use of rhodium acetate as a precursor results in a higher amount of atomically dispersed rhodium when compared with the respective samples synthesized with rhodium chloride. The addition of ceria to the support increases the dispersion of rhodium, independent of the metal precursor used. Finely tuned thermal treatments, careful choice of metal precursor and support type are conditions for the synthesis of stable atomically dispersed catalysts.
Acknowledgements The authors are grateful for the beamtime provided at the SuperXAS beamline, in the Swiss Light Source at the Paul
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Scherrer Institute, Villigen, Switzerland. We thank Urs Hartfelder, ´ Kopelent and Mattha ¨us Rothensteiner for Cristina Paun, Rene assistance with the XAS measurements. We also thank Bruce C. Gates, Cong-Yan Chen, Isao Ogino and Joseph D. Kistler for the provision of the crystallographic parameters and reference file of the Rh–Al scatter pair. The electron microscopy center of ETH Zurich (EMEZ) is acknowledged for providing measurement time.
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